Abstract:

In the canyon grasslands of Garden Creek Ranch Preserve in Idaho, where the
threatened plant Silene spaldingii occurs and invasion by the exotic species Centaurea
solstitialis and Bromus tectorum is proceeding rapidly, I examined environmental and
community patterns of site invasion, and evaluated the apparent influence of invasion on
Silene population vigor. In addition, two separate lightning fires at the preserve
presented the opportunity to examine the short-term influence of late-season fire on this
species and its associated bunchgrass plant community.
I found that Silene-supporting sites most often invaded by exotics were on
relatively gentle slopes that received more incident radiation. This pattern may relate, in
part, to light requirements of Centaurea solstitialis. Invaded sites were also typically at
higher elevations, which may indicate they were moister and therefore more productive.
The plant communities in invaded Silene-supporting sites were similar to plant
communities in uninvaded sites, although invaded sites tended to have greater legume
and exotic annual grass cover. Exotic species invasion did not appear to influence
negatively the vigor of Silene populations, as indicated by similar plant height and
comparable levels of flowering, fruit and seed set in invaded and uninvaded populations.
The similarity in Silene vigor between invaded and uninvaded sites may reflect a moderating influence of site productivity in invaded populations, or may indicate that
mature Silene plants and the exotic species partition space or resources differently, potentially reducing competition between them. However, Silene recruitment may be limited by competition from weeds; my data did not allow a rigorous test of this possibility.
Fire apparently decreased cover of Festuca idahoensis and increased cover of Lupinus sericeus in the first year after burning, while cover of Pseudoroegneria spicata, exotic grasses, and most other forb species did not differ between burned and unburned areas. Silene cover and abundance within populations were similar before and after fire. Burning did not appear to influence levels of flowering, change the number of flowers or capsules produced per stem, or alter the number of seeds per capsule. Burning decreased plant size slightly, and decreased the proportion of flowers that matured to seed-filled capsules. Silene and the plant communities that support this species appear well suited to late season fire, however the response to burning in other seasons or at higher frequencies remains unknown in this study area.